The fabrication of ceramic materials characterized by exceptional resistance to thermal shock, ablation, weather erosion, and the like has become a technology of significant importance. A number of materials have been suggested and tried in an attempt to find such a suitable material. For example, among the current conventional radome materials, alumina and "Pyroceram 9606", crystalline glass-like ceramic sold by Corning Glass, Inc., have been clearly demonstrated not to meet all these advanced requirements. Fused SiO.sub.2, while having adequate resistance to thermal stress fracture, has inadequate ablation resistance due to its limited refractory character, as well as significant weather erosion deficiencies. Si.sub.3 N.sub.4 has extreme thermal environment limitations, while boron nitride is extremely expensive as well as mechanically weak.
Important elements in improving a ceramic composite's thermal-stress resistance in extreme high-temperature environments are to reduce the thermal conductivity of the material and increase the strain tolerance. Introduction of a second phase material into the crystallline microstructure can significantly reduce the thermal conductivity of a ceramic material. The size and thermal conductivity of these second phase particles are extremely important composite parameters. In particular, the particle size and thermal conductivity of the second phase material determines the character of the immediate microstructure of the composite i.e. the magnitude and number of microcracks which result upon thermal expansion of both composite materials when exposed to significant temperature fluctuations. Generation of microcracks is an important strain accommodating mechanism, and hence increases strain tolerance.
The prior art has attempted to solve the problem of increased resistance to thermal shock by placing the emphasis on inhibiting or arresting crack propagation. One method of increasing the thermal resistance of a ceramic material has been the incorporation of small boron nitride particle into a matrix thereof, as described by Rice et. al. in U.S. Pat. No. 4,304,870, incorporated herein by reference. Nevertheless there have been difficulties encountered in prior art methods of incorporating BN powders into a ceramic matrix. BN powders are susceptable to oxidation. Because of their high surface area to volume ratio, small BN powders are especially susceptable to oxidation. Thus, a lower limit upon the size of BN powders which can be incorporated results. Also, due to their plate-like shape, BN particles orient during hot pressing. This orientation results in an anisotropy of properties, which may not be desirable for some uses. Further, prior art has required hot press temperatures of over 1700.degree. C., which has resulted in difficult and expensive production. Moreover, shrinkage during sintering has made precise molding of ceramic articles difficult.